Abstract

Like other floating-leaved plants, dense beds of water-chestnut (Trapa natans) may cause the underlying water to become hypoxic or even anoxic. Despite this potential stress, previous studies have reported dense and diverse invertebrate communities in water-chestnut beds. However, none of these previous studies simultaneously measured dissolved oxygen concentrations and invertebrate communities, leaving open the possibility that these study sites were not severely hypoxic. Here, we report the results of a study in which we measured both invertebrates and dissolved oxygen in a large, dense water-chestnut bed in the freshwater tidal Hudson River, New York. We sampled invertebrates at nine sites in the bed using a modified Downing box-sampler and a corer. We took samples both early in the growing season (June), when the water-chestnut canopy was not fully developed and hypoxia was only moderate, and in the middle of the growing season (July), when the canopy was fully developed and hypoxia was frequent and severe. We hypothesized that the effects of hypoxia would be more severe in July than in June, in the interior of the bed than near its edge, and for sediment-dwelling animals than for epiphytic animals. We also hypothesized that predatory invertebrates would be especially abundant in the most hypoxic sites, where fish predation presumably would be reduced. None of these hypotheses was supported. Instead, we found that all locations and microhabitats that we studied supported dense, diverse communities of invertebrates, including insects, oligochaetes, crustaceans, and other taxa. The mechanisms by which these animals tolerate short-term hypoxia need to be elucidated. We suggest that water-chestnut beds in the Hudson are valuable habitats for invertebrate biodiversity and production, and may contribute substantially to fish production.